Measurements of electrical impedance of the human body (bioimpedance) have been studied in bioengineering since the 1960s. These measurements include forcing an alternating current (AC) through the body (usually at a frequency higher than 10 kHz to avoid interference with the electrical activity of nervous and muscular tissues), and sensing the voltage drop between two points.
Water and generally all body fluids (blood, intra and extra cellular fluid, for example) provide the conductive medium of the body. Several measures and studies have been carried on applying this technique in different parts or regions of the body and using different frequencies to target different biological information (See, for example, Deok-Won Kim, Detection of physiological events by impedance, Yonsei Medical Journal, 30(1), 1989). In numerous applications the absolute value of the bioimpedance may be determined because it may be relatively simple to calculate it and it may provide much information. In other applications, both the modulus and phase of the complex bioimpedance may be measured.
It may be relatively difficult to determine relatively precise and reliable mathematical models of bioimpedance, particularly in thoracic regions. The main factors influencing electrical impedance in the chest may be the blood in the heart and in the aorta, and the pleural fluids and pulmonary circulation. Heart pumping, causing a variable distribution of blood in the heart-aorta region, and respiration, may be responsible for small variations of thoracic bioimpedance (i.e. the impedance of biological tissues). From these variations it may be possible to determine heart rate, breath rate, and evaluating cardiac output (volume of blood pumped by the heart for unity of time).
The measurements may be carried out using two or four electrodes, as schematically shown in FIG. 1. By using two electrodes, the measured impedance is the sum of the bioimpedance Zbody and of the contact impedance Ze at the electrodes. Generally, the impedance Ze disturbs the measure of the impedance Zbody. Using a four electrode setup, it may be possible to measure the impedance Zbody as the ratio between the measured voltage drop and the current forced through the body tissue with relatively more precision because the measurement may not be affected by the contact impedance Ze.
There may be a relatively strong interest in methods of carrying out this measure. Since it is a non-invasive technique, it may be correlated to a vast range of physiological parameters, thus, it may have a strong potential in many medical fields. Furthermore, the relative simplicity of the measurement, the integrability, the reduced size, and the low cost of the equipment, may make the technique of measuring thoracic bioimpedance particularly suitable to be implemented in wearable or implantable health monitoring systems.
The voltage VZ(t) sensed on the electrodes is an AC signal modulated by the bioimpedance Z(t):VZ(t)=Z(t)I0 sin(ωt)With an AM demodulator it may be possible to obtain a baseband signal representing the modulus |Z(t)| of the impedance. The phase of Z(t) may be evaluated, for example, by measuring the delay between the input current and output voltage or with a phase and quadrature demodulation.
A block diagram of a typical circuit for measuring the impedance of a biological tissue is illustrated in FIG. 2. An AC voltage generated by an oscillator is used to control a voltage-to-current converter that delivers a current Iz that is injected through the biological tissue using two or four electrodes. The voltage on the biological tissue is sensed, amplified, and AM demodulated for obtaining a baseband signal. The DC component Z0 and the AC component deltaZ of the baseband signal are extracted using a low-pass filter LPF and a high-pass filter HPF and converted into digital form by an analog-to-digital converter ADC.
A sinusoidal voltage may not be used, but it may be desirable to reduce the attenuation of higher harmonics due to capacitive effects and to use an envelope detector as an AM demodulator. Furthermore the use of an adjustable sinusoidal waveform may make frequency analysis and characterization of tissues possible.
This type of system may be characterized by the presence of an instrumentation amplifier (INA) upstream from the AM demodulator. A drawback of this signal processing path is that the INA works on the modulated input signal. For this reason, the known architecture of FIG. 2 generally requires either an INA of a sufficiently large bandwidth, and, thus, has a large current consumption, or use of a low frequency for the injected current. This is a limitation because INAs, especially low power consumption and low cost devices, usually have a relatively narrow bandwidth.
Another point of the architecture of FIG. 2 is the voltage-to-current converter. It may be desirable that this circuit have a relatively large output impedance to provide negligible variations of the amplitude of the injected current when the load varies, and be DC decoupled for safety reasons, because it is desirable that DC current forced through human body tissue be less than 10 μA under normal conditions and less than 50 μA in single fault condition (See, Association for the Advanced of Medical Instrumentation. Medical electrical equipment—Part 1: General requirements for basic safety and essential performance. ANSI/AAMI ES60601-1:2005).
As disclosed in Rafael Gonzalez-Landaeta, Oscar Casas, and Ramon Pallas-Areny, Heart rate detection from plantar bioimpedance measurements, IEEE Transactions on Biomedical Engineering, 55(3):1163-1167, 2008, another known measurement system is depicted in FIG. 3. AM demodulation is performed upstream from the INA to increase the Common Mode Rejection Ratio (CMRR). The circuitry is fully differential, and a differential stage with coupled amplifiers is used as the first stage of the voltage drop on the electrodes. A high pass filter HPF and amplifier stage are used for extracting the AC components of the signal, deltaZ, that, in many applications (for example, thoracic bioimpedance measurement) includes physiological information.
The bandwidth of such a system is limited by the coupled amplifiers stage. The higher the gain (that is, greater than one), the lower the bandwidth. The working frequency used in Rafael Gonzalez-Landaeta, Oscar Casas, and Ramon Pallas-Areny, Heart rate detection from plantar bioimpedance measurements, IEEE Transactions on Biomedical Engineering, 55(3):1163-1167, 2008, for example, is fixed at 10 kHz, which is relatively small.